The present application is related to the following applications even dated herewith: Ser. No. 09/478,563, entitled xe2x80x9cCommunications Network Systemxe2x80x9d to Cherubini, et al.; and Ser. No. 09/478,565, entitled xe2x80x9cHybrid TDMA/CDMA System Based on Filtered Multitone Modulationxe2x80x9d to Cherubini, both of which applications are incorporated by reference herein.
The invention concerns transmission techniques which are suited for use in digital subscriber line (DSL) systems, and in particular very-high-rate digital subscriber line (VDSL) systems.
Herein considered are transmission techniques for VDSL, an emerging network-access technology over telephone copper cables that can be viewed as an evolution of the asymmetric digital subscriber line (ADSL). [See: G. T. Hawley, xe2x80x9cSystems Considerations for the Use of xDSL Technology for Data Access,xe2x80x9d IEEE Commun. Mag., vol. 35, pp. 56-61, March 1997.]
ADSL was developed in the early 1990s to enable interactive video-on-demand service over the same twisted-pair used for analog telephony. The asymmetry in the transmission rates results from the fact that the delivery of compressed digital video signals in the downstream direction requires a much larger capacity than the transmission of the return signal in the upstream direction. According to the ADSL standard approved by the American National Standards Institute (ANSI) in 1995, downstream rates up to 6.176 Mbit/s and upstream rates up to 640 kbit/s are considered. Details of the ADSL standard are given in the reference xe2x80x9cNetwork and Customer Installation Interfacesxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface,xe2x80x9d ANSI T1E1.413-1995, August 1995.
The reference by M. Cioffi, xe2x80x9cAsymmetric Digital Subscriber Lines,xe2x80x9d in The Communications Handbook, ed. J. D. Gibson, ch. 34, pp. 450-479, CRC Press, 1997 is a tutorial on ADSL which is incorporated by reference in its entirety. More recently, the rapid expansion of Internet services and the potential interest of subscribers in services such as teleconferencing and telecommuting have motivated ANSI and the European Telecommunications Standards Institute (ETSI) to start work on VDSL, where rates up to 52 Mbit/s in the two directions of transmission are envisaged.
The version of orthogonal frequency division multiplexing (OFDM) known as discrete multitone (DMT) modulation was chosen by ANSI as a standard for ADSL transmission; see, J. A. C. Bingham, xe2x80x9cMulticarrier Modulation for Data Transmission: An Idea Whose Time Has Comexe2x80x9d IEEE Commun. Mag., vol. 28, pp. 5-14, May 1990, and J. S. Chow, J. C. Tu, and J. M. Cioffi, xe2x80x9cA Discrete Multitone Transceiver System for HDSL Applications,xe2x80x9d IEEE J. Select. Areas Commun., vol. 9, pp. 895-908, August 1991. The references to Bingham and Chow, et al. supra, are incorporated by reference in their entirety. In general, OFDM systems employ M orthogonal subcarriers for parallel transmission of blocks of M symbols over M subchannels. The chosen DMT system is well suited for ADSL transmission over telephone copper cables, because it allows more flexibility in capacity allocation and provides higher robustness in the presence of impulse noise than other passband techniques, e.g., carrierless amplitude and phase (CAP) modulation. The design of a VDSL transceiver, however, is more difficult than the design of an ADSL transceiver, because of the high rates considered. Echo and alien near-end crosstalk (NEXT) signals are the predominant interferences for VDSL transmission. Alien NEXT represents a problem mainly for upstream transmission, because twisted pairs are usually bundled in the same multipair UTP cable at the central office.
A new transmission technique related to OFDM is herein proposed for high-speed transmission over twisted-pair cables. The scheme, which is herein called xe2x80x9cfiltered multitone modulationxe2x80x9d (FMT), exhibits significantly lower spectral overlapping between adjacent subchannels and provides higher transmission efficiency than DMT.
Multitone modulation with a high level of subchannel spectral containment are mentioned in European Patent application bearing the title xe2x80x9cCommunication Network Systemxe2x80x9d, G. Cherubini and G. Ungerboeck, filed on the same day as the present application and currently assigned to the assignee of the present application and also in an article by J. Karaoguz, J. Yu, V. Eyuboglu, xe2x80x9cComparison of Single-Carrier, Multi-Carrier, and Spread Spectrum Modulation for Upstream PHY Layer in HFC CATV Networks,xe2x80x9d IEEE 802.14 contribution 98-018, July 1998. The references mentioned immediately above are incorporated by reference in their entirety. In the aforementioned Karaoguz, et al. reference, the filters are ideal unrealizable square-root raised cosine filters with zero roll-off. There is further proposed, methods of echo cancellation for FMT modulation, which allow full-duplex VDSL transmission over the entire bandwidth.